It may very well be beneficial for a parent to convert an IRA to a Roth IRA if the main concern is to pass as much income after tax and give the beneficiaries maximum flexibility.
Although you do not indicate the level of income of your mom, it is quite likely that she is in a lower tax rate than you and your brother. So, less tax will be paid by mom then would be due by you and your brother when distributions are taken.
For more information see What if You Inherit an IRA?
"If you inherit a traditional IRA from anyone other than your deceased spouse, you cannot treat the inherited IRA as your own. This means that you cannot make any contributions to the IRA. It also means you cannot roll over any amounts into or out of the inherited IRA. However, you can make a trustee-to-trustee transfer as long as the IRA into which amounts are being moved is set up and maintained in the name of the deceased IRA owner for the benefit of you as beneficiary.
Like the original owner, you generally will not owe tax on the assets in the IRA until you receive distributions from it. You must begin receiving distributions from the IRA under the rules for distributions that apply to beneficiaries."
See IRA Beneficiaries and Miscellaneous Rules for Required Minimum Distributions
An inherited IRA (except from a deceased spouse) can not be rolled into the beneficiary's IRA or the IRA of the beneficiary's children.
Whether the inherited IRA is a traditional or a Roth IRA that rule will apply.
The beneficiary of a traditional IRA must pay the income tax when distributions are made; but will owe no tax on distributions of a Roth IRA.
Contributions can be made to the individual's own separate IRA in the same year that distributions are required from the inherited IRA.
It seems that if the income from the IRA is split over future years and the individual dies prior to that future year that the income would have to be included in the year of death.
Please ask if you need clarification.